The invention relates generally to a semiconductor power device that controls AC power to a load, and more specifically to such a device having integral current monitoring.
In controlling the application of electrical power to a load, it is known practice to use a switching device in series with the load. Power MOS devices have proven to be very useful in this regard, at least for DC applications. The terms MOS and MOSFET (which originally stood for metal-oxide-semiconductor) are used to refer to insulated gate devices generally, notwithstanding the fact that most modern devices have polysilicon gates rather than metal gates.
AC switches using MOS technology are more complex in nature than DC switches, since AC switches need to conduct in both directions. It is known to create an AC switch by hooking up two MOSFET's as DC switches, either in a common drain or in a common source configuration. A somewhat simplified explanation for the AC operation of such a switch is that in a given polarity, one MOSFET conducts as a MOSFET while the other acts as a forward-biased diode in parallel with a MOSFET.
In most power electronics applications where AC power is controlled and delivered to a load, it is important to monitor and control the load current (which corresponds to the switch current) and the switch temperature. Certain properties of power MOSFET's make it possible to sense current, using the current mirror technique. Power MOSFET's are normally implemented as a plurality of discrete cells connected in parallel with gates in common, sources in common, and drains in common. The current mirror (or current sensor, as it is sometimes known) comprises a relatively small region of cells having their gates and drains in common with those of the main transistor and having their sources commonly connected to a separate output terminal. For example, the current mirror might comprise ten MOSFET cells while the main transistor comprised 10,000. The current flowing through the current mirror is found to be a predetermined fraction of the current flowing through the main transistor. If the mirror cells and the main cells are configured geometrically alike, the fraction corresponds roughly to the ratio of the numbers of cells (e.g., a current ratio of about 700:1 for a cell ratio of 1000:1).